1. Field of the Invention
The present invention relates to a technology for illuminating a pushbutton switch key sheet used for performing input on portable information terminals such as a mobile phone, a PDA, and a portable music player, and various electronic apparatuses such as an AV apparatus. In particular, the present invention relates to an illumination type key sheet, a lightproof light guiding sheet for use in the key sheet, a pushbutton switch provided with the key sheet, and a method of manufacturing the key sheet.
2. Description of the Related Art
Pushbutton switches for portable information terminals and various electronic apparatuses such as an AV apparatus usually adopt a structure in which a key sheet with pushbuttons (key tops) is placed on a circuit board on which contact switches are arranged. Further, in order to achieve enhanced operability in dark places, there is adopted backlight illumination, in which the key sheet is brightly illuminated with light from an inner light source provided on the back surface side of the key sheet.
As an example of such an illumination type pushbutton switch, JP 2008-140766 A discloses a structure in which a light guiding sheet (light guide) is provided between a circuit board on which contact switches (metal domes) are arranged and a key sheet (operation switch sheet) provided with key tops and a base sheet (elastic sheet). By thus providing a light guiding sheet, light can be passed through the light guiding sheet to be transmitted to the entire key sheet.
However, the structure disclosed in JP 2008-140766 A has a problem in that, due to the thickness of the light guiding sheet, the thickness of the pushbutton switch is so much the larger, that the number of components increases, and that the light guiding sheet held between the contact switches and the key sheet blunts the click feel, resulting in deterioration in operational feel. In view of this, there has been proposed an improved structure in which a highly transparent resin film is used as the base sheet and in which the base sheet also serves as the light guiding sheet, thereby avoiding an increase in the thickness of the pushbutton switch and the number of components and deterioration in operational feel.
In an illumination type pushbutton switch, a dark print layer is sometimes provided on the base sheet so as to cover the intervals between the key tops so that the light from the inner light source may not be allowed to leak through the periphery of the key tops. Provision of such a dark print layer in the above-mentioned improved structure involves provision of a dark print layer on the surface of the base sheet formed of a highly transparent resin film, and hence the dark print layer absorbs light transmitted to the interior of the base sheet, resulting in a reduction in the illumination luminance of the pushbutton switch.